1. Field of the Invention
The present invention relates to optical gyroscopes and more particularly to passive ring resonator gyroscopes which have bias and frequency errors resulting from mechanically or thermally induced dimensional changes.
2. Description of Prior Art
This application relates to five previously submitted applications, Ser. No. 676,322, "PASSIVE RING RESONATOR GYROSCOPE", filed 11/29/84, inventor Sanders et al; Ser. No. 701,891, "TWO SERVO LOOP PASSIVE RING LASER GYROSCOPE", filed 2/13/85, inventors SooHoo and Valle; Ser. No. 839,292, "TWO SOURCE LASER PASSIVE RING LASER GYROSCOPE", filed 3/13/86, inventor SooHoo; Ser. No. 864,232, "PHASE LOCKED PASSIVE RING LASER GYROSCOPE", filed May 19, 1986, inventor SooHoo; Ser. No. 28,833 "IDENTICAL SERVO FREQUENCY MODULATED PASSIVE RING LASER GYROSCOPE", forwarded Mar. 18, 1987, inventor SooHoo; and all having common assignee.
All five of these applications, describe a laser gyro having a single piece body incorporating a linear laser light source and a passive resonant cavity. The first application, Ser. No. 676,322, relies on three active servo loops for operation.
The second previous application, Ser. No. 701,891, describes a gyroscope having a single linear laser light source and a passive resonant cavity. This second gyro uses a first and second active servo loop for operation.
The third application, Ser. No. 839,292 describes a gyroscope having two laser sources. A first laser source produces a clockwise beam and a second laser source produces a counterclockwise beam. Both beams circulate in a sealed, evacuated passive cavity within the same body.
The fourth application, Ser. No. 864,232 describes a gyroscope using an external modulator to frequency modulate the input source beam and subsequently the detected clockwise and counterclockwise beams are demodulated at this same frequency to produce a more sensitive phase detection scheme.
The fifth application, Ser. No. 07/028,833, describes a gyroscope using an external modulator to frequency modulate the input beam, and the output beams are phase demodulated at this same frequency. In this application, two identical servo loops plus a cavity sum servo are used to create a more symmetric and sensitive servo system.
In a passive ring resonator gyroscope, a pair of monochromatic light beams counterpropagate about a closed-loop optical path, which forms a high Q resonant optical circuit. The stability of the path length between reflective surfaces forming the closed path is critical in maintaining resonance in the passive ring resonator cavity since dimensional changes contribute to bias frequency errors. A linear laser.sup.1 and a ring resonator to form a prior art passive ring resonator is depicted in an article by S. EZEKIEL and S. R. BALSAMO titled "A Passive Ring Laser Gyroscope", Applied Physics Letters, Vol. 30, No. 9, 1 May 1977, pg. 478-480. FNT For description of lasers and resonators refer to: Yariv, A., QUANTUM ELECTRONICS (John Wiley & Sons, 1975) or Sargent, M., et.al., LASER PHYSICS (Addison-Wesley Pub., 1974). A linear resonator is typically conceived as a linear or standing wave resonator with forward and backward waves in which a light completes an optical round trip by reflecting off a mirror and retracing its path. The forward and backward waves create a standing wave in the cavity. In a ring resonator, each light completes an optical round trip without retracing its path and hence the path encloses an area as shown in Ezekiel's paper.
In the passive ring resonator, such as that described in the EZEKIEL reference, two beams traveling in opposite directions around the closed-loop optical path are injected into the passive ring resonator from a single frequency light source. The single frequency light source for the passive resonator is typically an external linear laser. Spectra Physics Inc. of Sunnyvale, Calif. produces stabilized lasers with the required characteristics. As the ring resonator gyroscope cavity rotates in inertial space, the two counterpropagating beams travel unequal path lengths. This path difference, due to rotation in inertial space, give rise to a relative frequency difference (Sagnac effect.sup.2) between the two counterpropagating beams. .noteq.E. J. Post, "Sagnac Effect", Review of Modern Physics, Vol. 39, No. 2, April 1967, p. 475-493.
A ring resonator, as opposed to a linear resonator, can exhibit the Sagnac effect and detect inertial rotation. The relative frequency difference is detected as a changing interference fringe pattern which is then electronically interpreted to indicate the direction and inertial rate of rotation of the passive gyro about the gyro's sensitive axis. The sensitive axis of the gyro is along the direction normal to the plane of the passive resonator.
It is known that bias errors in the detected signal of a ring resonator gyro result from dimensional changes in the laser and in the passive ring resonator. Bias errors also result from Fresnel Drag; these errors arise from the presence of gases (e.g. air) in the path of the counterpropagating beams in the resonator. Bias errors are typically characterized as a frequency difference between the two light beams which is not related to the rotation rate. Bias errors are sometimes detected as a frequency difference in the absence of rotation or as post calibration changes in the frequency difference for a specific absolute inertial rotation rate.
The Passive Ring Resonator Gyroscope of the type described in the EZEKIEL reference is typically constructed by placing optical elements, such as mirrors, beamsplitters, etc. on an optical bench. The location, spacing and geometrical relationships between the elements of the gyro function to enhance the passive ring resonator gyroscope's sensitivity and stability. Experimental passive ring resonator gyroscopes, such as that described in the EZEKIEL reference, typically have path lengths of a few meters making them unsuitable for use as a navigational instrument. The large size of prior art passive ring resonator gyroscopes, such as that characterized in the EZEKIEL reference, also contributes to the likelihood of bias errors due to mechanical coupling and mechanical drift of the optical elements in response to physical and thermal forces acting on the laser and on the cavity optical table or bench.
U.S. Pat. No. 4,352,562 issued Oct. 5, 1982, inventor H. T. Minden, is related and of interest; however, this reference shows no tuning mechanism, and has a different frequency modulation scheme.